1. Field of the Invention
This invention relates to bleaching and especially to bleaching of high yield pulps with dithionite-based compositions. More particularly, it relates to handling pulps that have been reductively bleached with dithionite to obtain higher post-reversion brightness.
2. Review of the Prior Art
High yield wood pulps comprise groundwood pulps, either stone ground or refiner ground, and semi-chemical pulps. The former are respectively ground between stones or patterned steel discs without chemicals, and the latter are briefly softened by pressure cooking with chemicals before refiner grinding. These pulps are preferred for temporary usages requiring low strength, such as newsprint.
Groundwood pulps from certain woods, such as spruce, are sufficiently bright for such uses without bleaching, but others need to be increased in brightness, particularly if semi-chemically cooked. Dithionites, often termed hydrosulfites, are highly suitable for the purpose as a reductive bleaching or brightening agent. Zinc dithionite has been used for years, but it has been largely replaced by sodium dithionite because of ecological requirements.
By using a dithionite, the brightness of mechanically disintegrated woodpulp, which is normally in the neighborhood of about 60% versus an MgO standard, can be increased significantly, but reversion always occurs during handling of the bleached pulp so that the permanent increase in brightness is generally in the neighborhood of about 10 points or less.
U.S. Pat. No. 2,707,144 describe bleaching of mechanically disintegrated woodpulp with a water soluble dithionite in combination with a water soluble polyphosphate by mixing the dithionite and polyphosphate with the woodpulp at a temperature of 120.degree.-212.degree. F. and a pH of 3-8.5 while the consistency of the pulp is about 2-5%. As two examples, brightness increases of 4 points and 7.7 points for dithionite alone are given. The solution of bleaching material is intimately mixed with the pulp, and the mixture is then forced as a continuously moving stream, preferably characterized by streamlined plug flow, along a confined, air-tight path so that the flowing mixture of pulp and bleaching materials substantially fills the cross-section of the confined path and continues to flow in the closed circuit formed by this path until the desired bleaching is obtained.
U.S. Pat. No. 2,826,478 of Schucker teaches that bleaching of groundwood may be effected in or immediately following the grinding pit by mixing hydrosulfite and citrate and/or tetraborate under quiescent or relatively quiescent conditions and in confinement to minimize the possibility of entrainment of air.
In a paper presented at the annual meeting of the technical section, Canadian Pulp and Paper Association on Jan. 23-26, 1968, Fleury and Rapson presented data on reducing chromophoric groups in groundwood, without disrupting the macromolecular system of the wood, by use of uranium-III, chromium-II, dithionite, and borohydride as bleaching agents. They demonstrated that reduction with the dithionite anion increased its brightness, under any conditions of time, temperature, pH, or quantity of reducing agent, no more than 10 points under an inert gas atmosphere such as carbon dioxide or nitrogen since the oxygen in the air destroys the reducing chemicals, such as dithionite. Experimental reductions were made with uranium and chromium as cationic reducing agents, both at pH=1, and with dithionite and borohydride as anionic reducing agents, at respective pH conditions of 5.5 and 10.5. All reductions were carried out with the same equipment and always under an inert gas atmosphere such as carbon dioxide or nitrogen because the oxygen in the air destroys the reducing chemicals. An exemplary reduction with uranium-III was described, using 1.25 electron-equivalents. A carbon dioxide atmosphere was maintained during the bleaching operation at 60.degree. C. and during cooling to 20.degree. C. After filtering and thorough rinsing with water to remove most of the uranium, carbon dioxide was again used during suspension in an aqueous solution of 0.2N Na2 EDTA and heating at 85-95.degree. C. for 15 minutes and then during a repetition of this washing operation.
Hand sheets were made from the reduced groundwood and dried as quickly as possible under vacuum while excluding oxygen. Reflectance measurements were made against magnesium oxide as a 100% reflectance standard and evaluated over the range of 300-600 mu. The brightness gain of 20 points for the uranium-III bleaching was not permanent. After storage of the hand sheets at room temperature, the brightness dropped to about 16 points above the original pulp brightness within a few days.
U.S. Pat. No. 3,467,574 describes a process of bleaching lignin cellulose material with a reducing bleaching agent which utilizes multi-stage refiners. In the first refiner, the temperature of the material is increased to facilitate out-gassing of oxygen-containing vapors which are deleterious to reductive bleaching. Alternatively, the system may be maintained under a pressure less than atmospheric, so that lower temperatures are necessary to effect out-gassing, or inert gasses may be employed to minimize oxidation. After partial defiberizing in the first refiner at a temperature of about 70.degree.212.degree. F. and a consistency of 15-50%, the pulp is further refined in a second refiner stage and then conveyed along an enclosed passageway to a third refiner stage, with a reductive bleaching agent, such as zinc dithionite, being added thereto as it is fed to this stage.
The bleached pulp is discharged from the bleaching refiner into a sump from which air is substantially excluded while the consistency of the pulp is reduced to about 4-5%. The pulp is retained in the sump for about 20 minutes to fully develop maximum brightness.
The paper industry is presently requiring ever higher brightness standards, and dithionite reductive bleaching is having difficulty meeting these higher standards which are in competition with oxidative bleaching processes.